[FIR] fix false negative MUST_BE_INITIALIZED for deferred init + custom setter case

^KT-58346 Fixed
Review: https://jetbrains.team/p/kt/reviews/9967

BTW this commit accidentaly and partially fixes KT-57553 for K2, because
of a cleaner K2 architecture. I will unify MUST_BE_INITIALIZED behaviour
in K1 and K2 in the next commits

compiler/testData/diagnostics/tests/backingField/InitOpenSetter.fir.kt

@@ -2,7 +2,7 @@ abstract class My(val v: Int) {
     // Ok: variable is just abstract
     abstract var x: Int
 
-    open var y: Int
+    <!MUST_BE_INITIALIZED_OR_BE_ABSTRACT!>open var y: Int<!>
 
     <!MUST_BE_INITIALIZED_OR_BE_ABSTRACT!>open var z: Int<!>